Arena advertises 4 languages the judge can't run (C#, Go, Rust, Zig)

**Severity:** High · **Verified:** yes (end-to-end) The language picker offers 9 languages, but the judge only supports 7. Selecting **C#, Go, Rust, or Zig** and hitting Run/Submit returns `400 Unsupported language`. **Where** - `src/components/cp-arena/mockData.ts:8-29` — `LANGUAGES` lists cpp, python, java, c, **csharp**, javascript, **go**, **rust**, **zig**. - `src/lib/judge.ts:47-60` — `JUDGE_LANGUAGE` only maps cpp/c/python/javascript/typescript/sql/java. - `src/components/cp-arena/ArenaWorkspace.tsx:335` sends the raw id → `src/app/api/run/route.ts` rejects unknown ids. **Fix (pick one)** - Remove csharp/go/rust/zig from `LANGUAGES` + `STARTER_CODE` + the `LanguageId` union until the engine supports them, **or** - Add real support in the judge engine and `JUDGE_LANGUAGE`. **Acceptance:** every language shown in the picker actually runs and returns a verdict (no `400 Unsupported language`).